1. Field of the Invention
This invention relates to a sheet detecting device for detecting a transparent or non-transparent sheet.
2. Description of the Prior Art
In copying apparatus, microfilm cameras, microfilm readers or the like, a sheet-like original or a sheet such as microfilm is fed and detected by a detecting device and when the sheet has been detected, the feeding of the sheet is stopped or other operation of the apparatus is controlled.
This detecting device has been provided with a light emitting element and a light receiving element and has detected the presence of a sheet by one of the methods illustrated in FIGS. 1 to 3 of the accompanying drawings. In FIG. 1, reference numeral 1 designates a sheet moving in the direction of the arrow, reference numeral 2 denotes a light emitting element, and reference numeral 3 designates a light receiving element. The light emitted from the light emitting element 2 is reflected by the surface of the sheet 1 and the reflected light is caused to enter the light receiving element 3 and any variation in the quantity of light entering the light receiving element 3 is detected to thereby detect the presence of the sheet. This method suffers from no problem where the sheet moves while being maintained in a predetermined planar condition, but if the sheet is fed while being curved or with the surface thereof inclined relative to the horizontal plane, the direction of the reflected light will vary and the reflected light from the surface of the sheet will not properly enter the light receiving element 3 and this may lead to a problem that the light receiving element 3 does not properly detect the presence of the sheet although the sheet is placed at the predetermined detection position. In this case, if the detection level of the detecting circuit receiving the output signal of the light receiving element is reduced so that the presence of the sheet may be detected even when the quantity of light entering the light receiving element 3 has been reduced by the inclination of the surface of the sheet, the presence of the sheet can be properly detected even if the surface of the sheet is varied, but in that case, the device is liable to be affected by noise and there occurs a new problem that malfunction is caused. Also, in a case where a sheet having a diffusing surface such as paper is to be detected, the difference between the quantity of light entering the light receiving element 3 when the sheet is present at the detection position and the quantity of light entering the light receiving element 3 when the sheet is not present at the detection position is small and this also leads to a problem that malfunction is caused. In any case, there has been a problem that the device is affected by extraneous light and although the sheet is not present at the detection position, there is caused a malfunction that a sheet detection signal is put out from the light receiving element by mistake.
FIG. 2 shows a detecting device in which the light emitting element 2 and the light receiving element 3 are disposed in opposed relationship with the movement path of the sheet 1 interposed therebetween. In this detecting device, when the sheet 1 is not present between the light emitting element 2 and the light receiving element 3, the light from the light emitting element 2 enters the light receiving element 3, whereby the light receiving element 3 does not put out a sheet detection signal and, when the sheet 1 is present between the light emitting element 2 and the light receiving element 3, the light from the light emitting element does not enter the light receiving element 3, whereby the light receiving element 3 puts out a sheet detection signal. This method suffers from a difficulty that the sheet cannot be detected if it is a transparent film.
FIG. 3 shows a detecting device in which the light emitting element 2 and the light receiving element 3 are disposed at one side of the sheet transportation path and a reflecting plate 4 is disposed at the other side of the sheet transportation path. In this detecting device, the light receiving element 3 receives the light reflected by the reflecting plate 4. This method, like the method of FIG. 2, cannot detect transparent sheets.
As described above, none of the detecting methods of the prior art could reliably detect the presence of all of transparent sheets, non-transparent sheets, sheets having a diffusing surface and sheets of which the surface planarity and inclination angle are variable.